REM sleep is unique in that the brain's activity patterns closely resemble those during wakefulness, despite the person being asleep. During REM sleep, the brain exhibits fast, desynchronized brain waves, primarily beta waves, which are similar to those observed when a person is awake and alert. This heightened brain activity is in stark contrast to the slower, synchronized waves seen in non-REM sleep stages. However, despite the similarity in brain wave patterns, REM sleep is distinct from wakefulness in several ways. One of the most notable differences is muscle atonia or the paralysis of voluntary muscles, which occurs during REM sleep to prevent individuals from acting out their dreams. Additionally, the type of cognitive processing and memory consolidation that occurs during REM sleep is different from that in wakefulness. REM sleep is associated with the processing of emotional experiences and procedural memory consolidation, which is crucial for learning complex tasks and emotional regulation. Thus, while REM brain activity mirrors that of an awake state, the physiological and cognitive processes happening during this stage are unique to sleep.

It is theoretically possible for an individual to skip certain sleep stages, particularly under conditions of sleep deprivation or disorders that affect sleep architecture. However, skipping any sleep stage, especially the deep sleep of Stage N3 or REM sleep, can have significant implications. Each sleep stage serves specific physiological and psychological functions. For example, Stage N3, or deep sleep, is critical for physical restoration, immune system function, and memory consolidation. Missing this stage can lead to impaired cognitive function, increased susceptibility to illness, and overall feelings of fatigue. Similarly, skipping REM sleep, crucial for emotional processing and memory, can impact mood regulation, stress management, and mental agility. Chronic deprivation of REM sleep has been linked to increased anxiety, mood disorders, and reduced coping mechanisms. Overall, each stage of sleep is essential for different aspects of health and well-being, and missing any stage can disrupt the body's natural restoration processes, impacting both physical and mental health.

Age significantly impacts the sleep cycle, particularly in the duration and quality of various sleep stages. As individuals age, there are notable changes in their sleep architecture. For instance, infants spend a significant portion of their sleep in REM sleep, which is essential for brain development. However, as people age, the total time spent in REM sleep decreases. Additionally, older adults often experience a decrease in Stage N3 sleep, the deepest and most restorative sleep stage. This reduction can lead to lighter, more fragmented sleep and an increased propensity to wake up during the night. Furthermore, older adults may find it more challenging to fall asleep and may experience advanced phase syndrome, where they go to sleep and wake up earlier than they did in their younger years. These changes can result in reduced sleep quality and quantity, impacting overall health, cognitive function, and quality of life. Therefore, understanding these age-related changes in sleep patterns is crucial for addressing sleep-related issues in different age groups.

The circadian rhythm, an internal biological clock that regulates the 24-hour cycle of biological processes, plays a crucial role in the timing and organization of the sleep cycle, including the alternation between REM and non-REM sleep. This rhythm is influenced by external cues, such as light and temperature, and dictates the timing of various physiological processes, including sleep. Typically, the circadian rhythm aligns sleep patterns with the external environment, promoting wakefulness during the day and sleep at night. During a typical sleep cycle, non-REM sleep dominates the early part of the night, making up a larger proportion of sleep cycles in the first half of the night. As the night progresses, the duration of REM sleep increases in each cycle, becoming longest towards the morning. This distribution suggests a coordination between the circadian rhythm and sleep stages, ensuring that deep, restorative sleep (non-REM) occurs early in the night and more REM sleep occurs closer to the natural waking time. Disruptions in the circadian rhythm, such as those caused by shift work or jet lag, can lead to misalignments in the sleep cycle, affecting the quality and duration of both REM and non-REM sleep stages.

Environmental factors such as light and noise have a significant impact on the stages of the sleep cycle. Light is one of the most influential environmental cues for the circadian rhythm, which regulates sleep patterns. Exposure to light, especially blue light from screens, can suppress the production of melatonin, a hormone that promotes sleep, leading to difficulties in initiating sleep. This disruption can delay the onset of sleep and consequently affect the progression through the various sleep stages, potentially reducing the duration of deep sleep (Stage N3) and REM sleep. Noise, on the other hand, can cause awakenings or shifts to lighter stages of sleep. Even if noise does not fully awaken an individual, it can cause a shift from deep sleep to a lighter sleep stage, reducing the overall quality of sleep. Chronic exposure to noise during sleep can lead to fragmented sleep patterns, decreased sleep efficiency, and reduced time in restorative sleep stages. Therefore, controlling environmental factors like light and noise is crucial for maintaining healthy sleep patterns and ensuring that the sleep cycle progresses smoothly through its various stages.